No. 20–The Tides

After breaking sequence last month to address the timely topic of shrimp season, we now return to the series with our topic of the month being Tides.

Tides are at once fascinating and confounding subject matter and we could easily spend a year of Minutes in the world of physics that dictates the foot or so tides of the Florida Keys and the forty foot tides of the Bay of Fundy For present purposes, however, we’ll go as easy as we can on the physics as we explore the celestial and terrestrial forces that orchestrate the symphony of tides.

Newton’s law of gravitation tells us that every particle of mass in the universe attracts every other particle of mass with a force that is proportional to the product of their masses. The greater the mass of objects and the closer they are together, the greater the gravitational attraction. That, friends, is as complicated as we’ll get. Take Sir Isaac’s law along with the fact that the primary factor of influence on tides is the combined gravitational force of our own sun and moon and give a spin in your head before proceeding.

Head still spinning? Well, thankfully, so is the earth and it is the earth’s spinning motion that contributes to terrestrial tidal forces as you will later see.

Recalling that particles of mass exert gravitational forces in proportion to their relative size and distance, try to visualize the moon, earth and sun lined up next to each other. Now make the earth four times the size of the moon and the sun one hundred and ten times the size of the earth. It would seem by comparison that the sun would have the greater tidal influence, right? Not so. Remember, relative distance is also involved in the equation. So even with a mass 27 million times greater than that of the moon, the sun is yet relegated to second fiddle in the tidal orchestra.

Again, relative distance is the key. While incredibly larger, the sun is also 390 times more distant from earth than the moon. Without getting into the equations, let it be accepted that the tide generating force of the sun is roughly 46% that of the moon. O.K., there are probably some physicist out there wondering when I’m going to address centripetal force. Well it’s just to complicated to cover in a Minute, or even five. Suffice it to say that it is the product of the earth spinning on its axis and the orbiting moon’s gravitational force. Centripetal and gravitational forces of sun and moon combine to generate the tidal dome effect observed as more pronounced on the spring tides of the new and full moons.

Regardless of their name, spring tides occur year round in conjunction with the new and full moons. They give us the highest high and lowest low tides of the lunar month. Their opposite, neap tides, occur on quarter moons and bring the lowest high and highest low tides of the lunar month. A lunar month is roughly 29 ½ days with lunar days of approximately 24 hours and 50 minutes in length. You may observe the daily, weekly and monthly tide cycles by consulting a tide chart or by following the tide tables carried in coastal newspapers.

To date we have dealt with the waters of the estuary and the forces that move them. There is still a little more to cover regarding the tides. In the months to come we will begin to delve more deeply into the wondrous estuarine world around us.